If coupling (e.g. acoustic, electromagnetic, electrical, magnetic, etc.) is present between the inputs and outputs in a signal processing system, there is a risk of feedback effects occurring. An example of such an arrangement is a hearing aid as schematized in FIG. 1. The hearing aid can be represented as a digital system 1 located in a particular environment. The input is constituted e.g. by a microphone 2. The picked-up signal is, among other things, amplified and fed out again via an earpiece 3. Acoustic coupling takes place via a physical feedback path 4 from the earpiece 3 back to the microphone 2. As a result of the feedback, feedback whistle occurs if both the amplitude and the phase condition is met. Audible artifacts occur even if the above conditions are only barely met.
To suppress the feedback effects a method is known whereby the physical feedback path 4 is digitally simulated by means of an adaptive filter 5 which is fed by the earpiece signal. The earpiece signal in turn originates from the hearing aid's internal signal processing unit 6 which picks up the microphone signal and amplifies it, among other things. After filtering in the adaptive compensation filter the earpiece signal is subtracted from the microphone signal in an adder 7.
Two paths are therefore present in the system, the physically existing feedback path 4 and the compensation path digitally simulated via the adaptive filter 5. As the resulting signals of both paths are subtracted from one another at the input of the hearing aid, the effect of the physical feedback path 4 is ideally eliminated.
An important component in the adaptive algorithm for compensating the feedback path is its step-size control. This governs the speed with which the adaptive compensation filter 5 adapts to the physical feedback path 4. As there is no compromise for a fixed step size, this must be adapted to the situation in which the system currently finds itself. In principle, a large step size must be striven for in order to achieve fast adaptation of the adaptive compensation filter 5 to the physical feedback path 4. However, the disadvantage of a large step size is that perceptible signal artifacts are produced.
If a feedback situation is well sub-critical, the step size should be extremely small. If a feedback situation occurs, however, the step size should become large. This ensures that the algorithm adapts the adaptive compensation filter 5 only when its characteristic differs significantly from the characteristic of the feedback path 4, i.e. when re-adaptation is necessary. For this purpose, a feedback detector is required.
Patent specification DE 199 04 538 C 1 discloses a method for feedback detection in a hearing aid whereby a frequency band is defined, a first signal level is determined in the frequency band, the signal is attenuated on a signal transmission path of the hearing aid and a second signal level of the attenuated signal is determined in the frequency band. Feedback can be detected on the basis of the first and second signal levels determined. However, if the input signal level varies it is difficult to quantify the feedback. Another disadvantage is that an audible effect on the forward signal path is to be expected and also that only slow detection of the feedback takes place, as the bands are ideally examined consecutively.